1. Field of the Invention
The invention relates to increasing the surface of firetubes in which the products of combustion from a burner increase the temperature of the liquid surrounding the firetube. More specifically, the invention relates to an increase in the surface of a portion of the firetube by a series of tubes mounted through the portion with the products of combustion on the external side of the tubes and the liquid to be heated on the internal side of the tubes.
2. Description of the Prior Art
There appears to be little drama remaining in the development of the firetube art. As sources of heat, these units are now all too familiar as conduits of various sizes which are extended through liquids to be heated. A burner of fluid fuel is conventionally mounted to discharge products of combustion into one end of each conduit. The heat from the products of combustion is transmitted through the walls of the conduit. The amount of heat released into the firetube and conducted into the liquid on the external side of the firetube is largely a factor of the size of the surface provided by the configuration of the firetube.
Directly related to the heat released within the firetube, and subsequently absorbed by the external liquid, is the final temperature of the products of combustion discharged from the end of the firetube as a conduit. The lower the final temperature, the more complete the heat transfer. This final temperature is often referred to as the "stack gas temperature." Hereafter, the products of combustion may be also called the firetube gases.
The focus of attack on stack gas temperature has been to increase the surface area of the firetube. For example, longitudinal fins have been mounted on the inside of the firetubes. However, those fins inherently tend to stagnate the film of heat exchange medium at the surface of the firetube. Also there is a structural problem with the fins which arises from the large temperature gradient between the hot tip in contact with the firetube gases and the cold base which is connected to the firetube wall.
Of course, a fin could be mounted in a spiral form on the inside of the firetube. This design would keep the flowing medium dynamic. However, it is not feasible to manufacture helical fins mounted internal a firetube.
Not to be overlooked are the heat exchange sections separately fabricated and mounted in the discharge section of the firetube. The liquid to be heated by the firetube proper can be preheated by the discharging stack gas in these sections. In either event, these sections scavange, conserve, recycle the heat of the firetube stack back to the basic process and thereby increase the overall efficiency of the process.
The basic difficulty with the separate heat exchange sections is that a pump is required to move the heat exchange fluid through the section and back to the basic process. Not only does this approach require the pump, but its operating and safety control system with all their expense of service, repair, and replacement.
In summation, there is the problem of providing a constructive increase in the surface area of the firetube. Both the surface exposed to the product of combustion flowing within the tube and the surface exposed to the liquid external the tube should be increased. Internal fins of the tube either cannot be given an efficient form or be fabricated practically. Additionally, internal fins do not increase the surface exposed to the liquid to be heated. Further, there is the problem of flowing the fluids over the heat exchange surfaces without the addition of pumping means. The question remains, "How are the surfaces of the firetube exposed to both the internal and external heat exchange fluids to be increased without the addition of a heat exchange section and without the addition of pumping means?"